The long answer is : You must make sure you have understood how to create users with postfix (see postfix documentation), then make sure that dovecot is working correctly for those users (see dovecot documentation), at which point Roundcube is connecting via IMAP to your dovecot server.
You will have to specify the name or ip of your dovecot server in the configuration file of roundcube (the php files) and use your email/username and password on the login page
Thomas